1. Technical Field
The present invention relates to fuel delivery systems gas turbine engines, and in particular to combined liquid fuel and purge air valves for turbine engines.
2. Description of the Related Art
Gas turbine engines have a set of rotating turbine blades that compress air leading to a combustion chamber into which fuel is injected and ignited. Fuel is delivered through metering orifices to burners in the combustion chambers under pressure through fuel lines. Combustion of the fuel turns a downstream set of blades from which energy is extracted and which can also be used to drive the compressor blades. Gas turbines used in power generation have large diameter blades and an array of combustion cans each with several burner nozzles that ignite the fuel at light-off and sustain combustion during operation. Many of these are dual fuel turbines (e.g., F-Class gas turbines) in which sustained operation can be achieved by burning either a liquid or a gaseous fuel. Diesel and heavy distillate fuel oils are common liquid fuels in power generation applications.
The combustion cans of the turbine engine are high pressure and temperature environments. It is typical for the environment surrounding the combustion cans to reach temperatures of 400° F., and for the combustion chamber temperature to near 2,000° F. The liquid fuel is consumed at a rate of about 20 gallons per minute at a high fuel pressure of about 1200 psig. This extreme environment is very hard on the fuel control components of the turbine fuel system, particularly for dual fuel turbines in which the during sustained gaseous burn, the liquid fuel system remains inoperable for long periods of time. The primary concern is the formation of the coke, or the tarry deposits left after the distillate or volatile components of the fuel are driven off by heat, on the metering orifices and other working surfaces of the liquid fuel control components. Coke deposits arise primarily from the presence of residual fuel left in the fuel atomizer, burner nozzles, control valves, fuel manifolds and other components subjected to the high heat of combustion. Residual liquid fuel left in the liquid fuel control components during gaseous operation will begin to coke at temperatures of about 250-280° F. in the presence of oxygen, which are well under the combustion temperature.
To evacuate the residual fuel, the fuel valves, burner nozzles and other components are purged with purge air bled from the turbine compressors or from an independent source. While the purge air coming from the compressors is hot, it is cooler than the combustion chamber temperatures so that it also cools the burner nozzles. Thus, air purging is necessary to prevent the burner nozzles from being damaged as well as to ensure that the system orifices and valves are clear of such deposits which could inhibit proper conduit of the fuel when the engine is returned to fuel mode.
In order to permit purging, a purging air line must join with the fuel line. It is important that the fueling and purging operations be isolated so that fuel does not go down the air purge line and hot gases do not travel up the fuel line to the fuel supply. Check valves are often installed in each of these lines to prevent this, thereby complicating the assembly and creating additional locations for coking to occur. And, common spring-loaded checks may be unreliable, sticking in the open position or allowing some backwash into either of the fuel or purge lines. Alternatively, the fuel cut-off valve may be operated by a pneumatic actuator in line with the check valve of the fuel line, however, this is also subject to coking and may introduce substantial pressure drops in the fuel flow. The fuel cut-off valve also introduces a potential failure point to the turbine where, if pressure is lost to the pneumatic actuator, the turbine will cease operating.
An effective three-way purge valve is disclosed in U.S. Pat. No. 6,050,081, assigned to the assigned of the present invention and hereby incorporated by reference as though fully set forth herein. That valve has a spool valve that shuttles between positions alternatively blocking the combustion can(s) from either the purge air line or the fuel line. The spool is biased to close off the fuel line and is urged to open the fuel line by a pilot air actuated piston. Thus, when fuel is to be closed off from the engine, the spool valve will return to its initial position thereby allowing the burner nozzles and the downstream side of the spool to be purged to reduce or eliminate coking in these areas.
Even with good air purging, coking can still be a problem. Not all of the components in the fuel system subjected to high temperatures, for example fuel check and cut-off valves, are often not able to be purged because they may be providing the barrier between the fuel and air streams. And, even those components that are air purged, some amount of residual liquid fuel may remain in the small internal chambers and passageways of the valves or other components of the fuel system. Moreover, since the air purge control valves may themselves be located at or near the combustion cans, they too are subject to the adverse affects of coking.
To further reduce the effects of coking, U.S. Pat. No. 6,729,135 discloses a system and method of circulating the liquid fuel through a heat exchanger to cool the temperature of the liquid fuel distillate during operation of the turbine in gaseous fuel mode. Coking is thus reduced in large part by keeping much of the liquid fuel distillate temperature below the coking threshold temperature. However, this system requires a heat exchanger and either a separate fuel recirculation pump or increased duty on the main fuel pump. Moreover, because the recirculation lines carry liquid fuel, these lines, along with any recirculation control components, present yet another location for coking to occur when the recirculation system is not operating. To avoid this, during liquid fuel operation some of the liquid fuel must be made to bypass the combustor to flow through the recirculation system. Intermittent operation of the recirculation system is also possible, but only to the extend that sufficiently cool fuel temperatures can be maintained.
U.S. Pat. No. 6,931,831 discloses an improved distributor purge valve that combines in one unit the pilot air, fuel metering, purging, and distributing components of the fuel system, and reduces coking problems by eliminating components where residual fuel can remain and by more effective use of the purge air within the valve. And, U.S. patent publication No. 2005/0097880 discloses such an combined valve construction with an integral water jacket containing coolant for cooling critical components where residual fuel may reside. Both U.S. Pat. No. 6,931,831 and the published application 2005/0097880 are hereby incorporated in their entirety by reference as though fully set forth herein.
While these more recent developments address the coking problem, there still remains the issue of the fuel system being compromised by the effects of the harsh environment and even the purge air itself. Specifically, for one thing, because of the close proximity of the fuel and purge air circuits, there is the possibility for leakage of the pressurized purge air into the fuel system. This can adversely affect the upstream fuel components as well as cause air pockets in the fuel that can disrupt performance of the engine and cause a lag in transitioning to fuel operation mode. Another problem is that the pressure of the fuel left inside the valve during purge mode operation can fluctuate due to thermal expansion or contraction, thereby creating a high pressure situation that must be relieved or a reduction available fuel in the valve, which leads to the type of issues described above regarding air being present in the fuel lines.
Accordingly, an improved fuel and purge valve arrangement is needed.